Process for producing alsimgcu alloy products with improved resistance to intercrystalline corrosion

ABSTRACT

The invention concerns a process for the production of rolled or extruded products of high strength AlSiMgCu aluminium alloy with good intergranular corrosion resistance, comprising the following steps: 
     casting a plate or billet with the following composition (by weight): 
     Si: 0.7-1.3% 
     Mg: 0.6-1.1% 
     Cu: 0.5-1.1% 
     Mn: 0.3-0.8% 
     Zr: &lt;0.20% 
     Fe: &lt;0.30% 
     Zn: &lt;1% 
     Ag: &lt;1% 
     Cr: &lt;0.25% 
     other elements: &lt;0.05% each and &lt;0.15% in total remainder: aluminium; with: Mg/Si&lt;1 
     homogenising in the range 470° C. to 570° C.; 
     hot working, and optionally cold working; 
     solution heat treating in the range 540° C. to 570° C.; 
     quenching; 
     annealing, comprising at least one temperature plateau in the range 150° C. to 250° C., preferably in the range 165° C. to 220° C., the total period measured as the equivalent time at 175° C. being in the range 30 h to 300 h.

FIELD OF THE INVENTION

The invention concerns high strength AlSiMgCu aluminium alloy productsdesignated by the 6000 series of the international nomenclature of the"United States Aluminum Association", for structural applications, inparticular in the aeronautical industry.

DESCRIPTION OF RELATED ART

Some alloys in the 6000 series have superior properties which renderthem suitable for the most demanding structural applications.

Thus United States patent U.S. Pat. No. 4,082,578 from ALCOA describestwo families of alloys, subsequently registered with the AluminumAssociation and designated 6009 and 6010, the first with superiorformability and the second with superior mechanical strength. Thesealloys have good dent resistance, stress corrosion resistance andexfoliation resistance, as well being well suited to resistance spotwelding. They are thus particularly suitable for automobile construction(bodywork and bumpers).

These alloys have the following composition (by weight):

Si: 0.4-1.2%

Mg: 0.4-1.1%

Cu: 0.1-0.6%

Mn: 0.2-0.8%

Fe: 0.05-0.35%

In some cases, in the T6 temper (in the Aluminum Associationdesignation), an ultimate tensile strength R_(m) of 400 MPa and a yieldstrength of 370 MPa at 0.2%, R₀.2, can be exceeded.

U.S. Pat. No. 4,614,552 from ALCAN concerns aluminium alloy sheets, alsofor automobile bodywork, with the following composition:

Si: 0.60-1.0%

Mg: 0.62-0.82%

Cu: 0.65-0.79%

Mn: 0.10-0.50%

Fe: <0.4%

Ti: <0.10%

Others: <0.05% each and <0.15% in total.

This alloy was subsequently registered under designation AA 6111. Incommon with alloys 6009 and 6010 above, it does not have good resistanceto intercrystalline corrosion in the T6 temper.

U.S. Pat. No. 4,589,932 from ALCOA proposes an alloy for automobile,rail, naval or aeronautical construction which was subsequentlyregistered under designation AA 6013, with the following composition:

Si: 0.4-1.2% preferably: 0.6-1%

Mg: 0.5-1.3% preferably: 0.8-1.2%

Cu: 0.6-1.1%

Mn: 0.1-1% preferably 0.2-0.8%

Fe: <0.5%

Cr: <0.10%

Ti: <0.10%

Zn: about 0.25%

The alloy is solution heat treated at 549° C. to 582° C., thistemperature being close to the solidus temperature.

The sheets obtained compare vary favourably as regards yield strengthand toughness, with coated alloy 2024 which is currently used foraircraft fuselages. Further, the manufacturing costs are lower.

However, some studies published in the scientific press have shown thatthis alloy has a high sensitivity to intercrystalline corrosion in theT6 temper (see T. D. Burleigh, "Microscopic Investigation of theIntergranular Corrosion of 6013-T6", in ICAA3, Trondheim 1992, p 435).

Our European patent EP-A-0 173 632 concerns extruded or forged productsof an alloy with composition:

Si: 0.9-1.3% preferably: 1-1.15%

Mg: 0.7-1.1% preferably: 0.8-1%

Cu: 0.3-1.1% preferably 0.8-1%

Mn: 0.5-0.7%

Zr: 0.07-0.2% preferably 0.08-0.12%

Fe: <0.30%

Zn: <0.7% preferably 0.3-0.6%

which has an essentially non re-crystallised structure.

That alloy, subsequently registered under designation AA 6056, has verygood mechanical properties for both strength and ductility:

    R.sub.m >420 MPa R.sub.0.2 >380 MPa A>10%

Our studies have shown that this alloy is also sensitive tointercrystalline corrosion in the T6 temper, with analogous results tothose of 6013 (see M. Reboul et al., "Stress Corrosion Cracking of HighStrength Al Alloys), in ICAA3, Trondheim 1992, p 455).

SUMMARY OF THE INVENTION

It has been noticed that the use of a particular region within thecomposition range of 6000 alloys containing Si, Mg and Cu, combined witha particular intercrystalline corrosion desensitising treatment, canproduce both mechanical properties equivalent to those of alloy 2024 inthe T3 temper and a considerably improved resistance to intercrystallinecorrosion in the non coated temper, meaning that alloys of this typetreated in this fashion are particularly suitable for the production ofaircraft fuselages and, more generally, to high strength structuralapplications.

The invention thus provides a process for the production of wroughtproducts of high strength AlSiMgCu aluminium alloy with goodintercrystalline corrosion resistance, comprising the following steps:

casting a plate or billet with the following composition (by weight):

Si: 0.7-1.3%

Mg: 0.6-1.1%

Cu: 0.5-1.1%

Mn: 0.3-0.8%

Zr: <0.20%

Fe: <0.30%

Zn: <1%

Cr: <0.25%

Ag: <1%

other elements: <0.05% each and <0.15% in total remainder: aluminium;with: Mg/Si<1

homogenising said plate or billet at a temperature which is in the range470° C. to 570° C.;

hot working, and optionally cold working;

solution heat treating at a temperature which is in the range 540° C. to570° C.;

quenching;

annealing, comprising at least one temperature plateau in the range of150° C. to 250° C., preferably in the range 165° C. to 220° C., for aperiod which is in the range 30 h to 300 h, preferably in the range 70 hto 120 h, measured as an in equivalent period at 175° C.

Preferably, annealing comprises a further temperature plateau at ahigher temperature which is in the range 185° C. to 250° C., theequivalent period at 175° C. always being in the range of 30 h to 300 hfor the total of the two plateaux.

The invention also provides a rolled or extruded aluminium alloy productwith the composition mentioned above, which is desensitised tointercrystalline corrosion (in the sense of the U.S.A Defense Departmentstandard MIL-H-6088) and, in the desensitised temper, with an electricalconductivity which is at least 0.5 MS/m greater than that measured forthe T6 temper.

The invention also provides an aircraft fuselage element or a road orrail vehicle structural element formed from the products of theinvention or products manufactured using the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Alloys of the invention having a Mg/Si ratio of <1 have a rather highersilicon content since the Mg composition ranges are typical series 6000alloys. It is surprising to obtain better intercrystalline corrosionresistance by increasing the Si content, since this is reputed to havethe opposite effect. Thus Kemal Nisancioglu in SINTEF Report A 820/3 of23/8/1982, "Intercrystalline, stress and exfoliation corrosion of AlMgSialloys. A literature survey", ISBN 82-0595-2860-6, p. 7, mentions that"the tendency towards intercrystalline corrosion (in the T6 temper)increases with the Si content, especially for alloys in which Si is inexcess with respect to the stoichiometric content".

It has been shown that with alloys in the same composition ranges, butwith a Mg/Si ratio of >1, the special anneal does not producesatisfactory desensitisation to intercrystalline corrosion. In fact,traces of localised intercrystalline attack are observed.Desensitisation could doubtless be obtained, but at the cost of anunacceptable degradation in mechanical properties.

In alloys of the invention with a Mg/Si ratio of <1, desensitised tointercrystalline corrosion, numerous intergranular precipitates havebeen observed which are in the form of platelets, while these are moreneedle-like in shape in the T6 temper. At least some of these plateletshaped precipitates contain quaternary AlMgSiCu compounds.

Further, the desensitised alloys of the invention have an electricalconductivity which is at least 0.5 MS/m higher than the electricalconductivity in the T6 temper when the anneal which is carried outcontains two plateaux, and by 1 MS/m when one plateau is employed.

The Cu content must be >0.5% for the alloy to have both sufficientmechanical properties and good thermal stability. Beyond 1.1%, there isa risk of stress corrosion problems and exfoliating corrosion appearing,thus reducing toughness, due to primary copper particles.

Addition of Zn in an amount which is in the range 0.15 to 1% has apositive influence on the intercrystalline corrosion resistance for anidentical composition and anneal. Further, addition of of the order of0.5% of Ag improves the mechanical properties.

The products of the invention can be rolled sheets or extruded profiles.The alloy is cast into plates (for sheets) or billets (for profiles) andthe transformation procedure is relatively conventional until the finalanneal. Homogenisation is carried out between 480° C. and 570° C. for aperiod which is in the range 5 to 50 h. Working by hot rolling orextruding, followed by cold rolling (for sheets) is then carried out toa thickness which is in the range 0.5 to 15 mm. Solution heat treatmentis then carried out at a temperature which is close to the solidus, inthe range 540° C. to 575° C., then water quenching at a cooling ratewhich depends on the thickness of the product.

The anneal is a particular heat treatment which produces both therequired mechanical properties and desensitises the alloy tointercrystalline corrosion. This treatment can be either asingle-plateau treatment at a temperature which is in the range 150° C.to 250° C., preferably in the range 165° C. to 220° C., or a two-plateautreatment, one of the plateaux being at a temperature which is in therange 150° C. to 250° C. (preferably 165° C. to 220° C.) and the otherat a higher temperature, in the range 170° C. to 270° C.

The treatment period depends on the temperature. This period can berelated to an equivalent period at 175° C., t_(eq), linked to thetemperature T of the plateau in °K and to the period t of treatment atthat temperature (the temperature rise period being taken into accountin the equivalent time calculation) by the relationship:

    (t.sub.eq /448) exp(-Q/448R)=t/T exp(-Q/RT)

where Q=145000 J/mol and R is the molar gas constant.

For two-plateaux treatments, it has been shown that desensitisation tointercrystalline corrosion is partial for t_(eq) >30 h and complete fort_(eq) >70 h. The term "partial desensitisation" means the absence ofintercrystalline dendrites with a length of more than 20 microns in apolished cut carried out following the test carried out in accordancewith American military standard ML-H-6088. Desensitisation is consideredto be complete for an absence of dendrites which are over 5 microns insize.

An equivalent period of more than 120 h is not recommended asdegradation of the yield strength is too severe, as it dropssubstantially below 300 MPa. The optimum for the desensitisation plateauis between 70 h and 120 h for two-plateau treatments and between 150 and250 h for single-plateau treatment. Following annealing, theconductivity is always more than 0.5 MS/m higher than in the T6 temper.

A single-plateau heat treatment can also be carried out. However, to beeffective, it must have an equivalent period which is longer than thatfor a two-plateau treatment, which generally leads to inferiormechanical properties. This equivalent period is preferably in the range150 h to 250 h. In this case, the conductivity is at least 1 MS/mdifferent from that of the T6 temper.

The products of the invention have a good yield strength and anexcellent specific strength (ratio of strength over density), takinginto account the fact that they have a lower density than that of 2000alloys, for example. Thus for 1.6 mm thick sheets, a strength of 71 GPawas measured, barely less than the module for sheets of the samethickness of bare 2024 alloy, and substantially superior to that ofcoated 2024 which is normally used for the fuselage of commercialaircraft.

Because of a high temperature anneal, these products also have goodthermal stability which makes them suitable, for example, for use in thefuselages of supersonic aircraft.

EXAMPLES Example 1

An alloy plate was produced with the following composition:

Si: 0.79%

Mg: 0.94%

Cu: 1.0%

Mn: 0.58%

Fe: 0.22%

Zn: 0.15%

giving a Mg/Si ratio of 1.2.

The plate was homogenised for 21 h at 530° C., scalped then hot rolledand cold rolled to a thickness of 1.6 mm Solution heat treatment wascarried out at 550° C. for 1 h.

A standard anneal for such an alloy, carried out in the T6 temper, wouldhave taken 8 h at 175° C. and the transverse mechanical properties inthis case were:

yield strength R₀.2 =375 MPa

ultimate tensile strength R_(m) =417 MPa

elongation A=14%.

The electrical conductivity was 24.0 MS/m.

Different heat treatments were carried out on these sheets to attempt todesensitise them to intercrystalline corrosion. This sensitivity wasqualified by using either an "Interneutral" test corresponding toAmerican military standard MIL-H-6088, or an internal test known as the"Interano" test, consisting of anodic attack of a sample for 6 h in achloride-perchlorate medium and at a current density of 1 mA/cm²,followed by micrographical examination.

The equivalent anneal temperatures and the results for the mechanicalproperties in the transverse direction and for intercrystallinecorrosion are shown in Table 1.

Example 2

Two alloys, A and B, were produced with the following composition:

    ______________________________________                  A    B    ______________________________________    Si:             0.95   0.82    Mg:             0.87   0.80    ______________________________________

Cu: 0.80 1.0

Mn: 0.63 0.58

Fe: 0.20 0.21

Mg/Si: 0.91 0.98

The plates were homogenised for 21 h at 530° C., scalped then hot andcold rolled to a thickness of 1.6 mm. Solution heat treatment wascarried out at 550° C. for 1 h for alloy A and at 570° C. for 1 h foralloy B. The standard anneal to produce the T6 temper was 8 h at 175° C.and the transverse mechanical properties were as follows:

For A R₀.2 =350 MPa R_(m) =380MPa A=13%

For B R₀.2 =363 MPa R_(m) =400 MPa A=14%

The conductivities in the T6 temper for alloys A and B were respectively24.3 and 24.7 MS/m.

Different heat treatments were carried out on these sheets to attempt todesensitise them to intercrystalline corrosion. This sensitivity wasqualified using the "Interneutral" and "Interano" accelerated tests.

The equivalent periods at 175° C., the transverse mechanical properties,electrical conductivity and sensitivity to intercrystalline corrosionare shown in Table 2 (for alloy A) and Table 3 (for alloy B).

Example 3

An alloy plate was produced with the following composition:

Si: 0.924

Mg: 0.860

Cu: 0.869

Mn: 0.550

Fe: 0.192

Zn: 0.152

Zr: 0.103

Ni: 0.017

Ti: 0.020

Cr: 0.004

giving a Mg/Si ratio of 0.93.

The plate was homogenised at 530° C., scalped then hot and cold rolledto a thickness of 35 mm. Solution heat treatment was carried out at550°, followed by quenching. Samples which had undergone conventionalannealing corresponding to a T6 temper were compared with samples whichhad undergone the intercrystalline corrosion desensitisation treatmentof the invention, with a two-plateau anneal of 6 h at 175° C.+2 h at220° C.

The mechanical properties, measured in the longitudinal andtransverse-longitudinal directions, were as follows:

    ______________________________________            L direction  T-L direction            R.sub.0.2                 R.sub.m  A      R.sub.0.2                                        R.sub.m                                             A            MPa  MPa      %      MPa    MPa  %    ______________________________________    T6 temper 368    380      13.0 356    394  9.6    of invention              315    344      11.5 316    349  9.0    ______________________________________

In the "Interano" and "Interneutral" tests, the samples which had beentreated in accordance with the invention exhibited an absence ofsensitivity to intercrystalline corrosion, in contrast to the T6samples,.

The rolled or extruded and intercrystalline corrosion desensitisedproducts of the invention are particularly suitable for the productionof structural elements for aeronautics, in particular fuselages, and forroad and rail vehicles.

                  TABLE 1    ______________________________________    HEAT      t.sub.eq                      R.sub.0.2                              R.sub.M                                    A     IC    TREATMENT (h)     (MPa)   (MPa) (%)   SENSITIVITY    ______________________________________    6 h 175° C. + 30              9.7     367     396   12.7  yes    min 200° C.    6 h 175° C. + 2 h              20.8    363     386   11.9  yes    200° C.    6 h 175° C. + 8 h              65.2    330     371   11.5  yes    200° C.    6 h 175° C. + 30              21.8    326     379   11.8  yes    min 220° C.    6 h 175° C. + 2 h              69.3    314     363   11.8  yes    220° C.    6 h 175° C. + 30              119.4   304     348   11.3  partial    min 250° C.    6 h 175° C. + 2 h              459.5   277     328   10.7  partial    250° C.    100 h at l75° C.              100     351     380   13    yes    8 h at 185° C.              18.3    360     398   6.7   yes    8 h at 220° C.              253.3   290     343   6     yes    ______________________________________

                  TABLE 2    ______________________________________                                         IC    HEAT      t.sub.eq                      R.sub.0.2                              R.sub.M                                    A    SENSI-                                               σ    TREATMENT (h)     (MPa)   (MPa) (%)  TIVITY                                               MS/m    ______________________________________    6 h 175° C. + 4 h              35.6    322     370   11.4 yes   24.6    200° C.    6 h 175° C. + 8 h              65.2    319     361   10   partial                                               24.7    200° C.    6 h 175° C. + 30              21.8    338     376   11.4 yes   24.5    min 220° C.    6 h 175° C. + 2 h              69.3    310     349   10.1 no    25.1    220° C.    6 h 175° C. + 30              119.4   288     331   10.1 no    25.8    min 250° C.    6 h 175° C. + 2 h              459.5   241     300   10.2 no    26.7    250° C.    8 h at 185° C.              18.3    349     388   11.1 yes   24.3    8 h at 200° C.              59.2    322     353   10.3 partial                                               24.7    8 h at 200° C.              253.3   272     323   9.5  no    25.8    ______________________________________

                  TABLE 3    ______________________________________                                         IC    HEAT      t.sub.eq                      R.sub.0.2                              R.sub.M                                    A    SENSI-                                               σ    TREATMENT (h)     (MPa)   (MPa) (%)  TIVITY                                               MS/m    ______________________________________    6 h 175° C. + 2 h               69.3   313     374   11   partial                                               25.1    220° C.    6 h 175° C. + 30              119.4   282     345   11   no    25.4    min 250° C.    ______________________________________

What is claimed is:
 1. A process for the production of high strength AlSiMgCu aluminium alloy products with good intergranular corrosion resistance, comprising the following steps:casting a plate or billet with the following composition (by weight):Si: 0.7-1.3% Mg: 0.6-1.1% Cu: 0.5-1.1% Mn: 0.3-0.8% Zr: <0.20% Fe: <0.30% Zn: <1% Ag: <1% Cr: <0.25%other elements: <0.05% each and <0.15% in total remainder: aluminium; with: Mg/Si<1 homogenising in the range 470° C. to 570° C.; hot working, and optionally cold working; solution heat treating in the range 540° C. to 570° C.; quenching; annealing, comprising at least one temperature plateau in the range 150° C. to 250° C., for a total period, measured as an equivalent period at 175° C., in the range about 59 to 300 h.
 2. A process according to claim 1, wherein Zn is present in a range of 0.15% to 1%.
 3. A process according to claim 1, wherein the annealing comprises a plateau at said at least one temperature which is in the range 150° C. to 250° C., and a further plateau at a higher temperature which is in the range 170° C. to 270° C.
 4. A process according to claim 3, wherein the equivalent period at 175° C. is in the range 30 h to 120 h.
 5. A process according to claim 4, wherein the equivalent period at 175° C. is in the range 70 h to 120 h.
 6. A process according to claim 1, wherein the annealing comprises a single plateau and its equivalent period at 175° C. is in the range 150 h to 250 h.
 7. A rolled or extruded product of high strength AlSiMgCu aluminium with the following composition (by weight):Si: 0.7-1.3% Mg: 0.6-1.1% Cu: 0.5-1.1% Mn: 0.3-0.8% Zr: <0.20% Fe: <0.30% Zn: <1% Ag: <1% Cr: <0.25%other elements: <0.05% each and <0.15% in total with: Mg/Si<1, which has been desensitised to intercrystalline corrosion within the meaning of standard MIL-H-6088, and has an electrical conductivity which is at least 0.5 MS/m higher than that measured for said composition in T6 temper.
 8. An aircraft fuselage element formed from rolled or extruded products produced by a process according to claim
 1. 9. An aircraft fuselage element formed from rolled or extruded products according to claim
 7. 10. A structural element for a rail or road vehicle produced from rolled or extruded products produced by the process of claim
 1. 11. A structural element for a rail or road vehicle formed from products according to claim
 7. 12. A process according to claim 1, wherein said at least one temperature plateau is in the range of 165° C. to 220° C.
 13. A process according to claim 3, wherein said plateau is in the range of 165° C. to 220° C. 